1. Field of the Invention
This invention relates to an auto-focus apparatus, image-capture apparatus, and auto-focus method, in which the distance measurement result of a distance measurement sensor and an evaluation value obtained from image processing are combined for use in automatically adjusting focus on a subject, so that the subject image is in focal status (herein after called the “in-focus”).
2. Description of the Related Art
Image capture devices such as video cameras and digital still cameras include automatic focus adjustment (auto-focus or “AF”) functions to automatically adjust the focal position to a subject. Various technologies have been proposed to improve the accuracy of such auto-focus functions.
For example, Japanese Unexamined Patent Application No. 2005-92085 discloses a focus detection and adjustment device that has phase difference detection-type focus adjustment unit and contrast detection-type focus adjustment unit, and which detects the difference in focus positions for the two focus adjustment unit and updates a conversion coefficient used in the phase difference detection-type focus adjustment unit. Through this configuration, the focus adjustment accuracy can be improved easily without any effort by a user capturing the image.
Further, the configuration of another video camera used is described. FIG. 1 shows the overall configuration of another video camera. This video camera combines a distance measurement sensor and an image processing AF function, and performs auto-focus operations. The lens block of the video camera shown in FIG. 1 includes a lens group having an image-capture lens 101c and a focus lens 101, as well as a position detector 101a, a lens drive mechanism 101b, and a lens driver 102. Also, the camera block includes an image-capture device 103, image-capture device driver 104, image signal generator 105, image signal processor 106, evaluation value calculator 107, control unit 109, memory 110, and distance measurement sensor 111.
In the video camera shown in FIG. 1, a subject image, with focus adjusted through the focus lens 101, is formed on a CCD (Charge Coupled Device) or other image-capture device 103, and after photoelectric conversion by the image-capture device 103, electrical signals are output to the image signal generator 105. The focus lens 101 is moved by the lens drive mechanism 101b, which receives instructions from the lens driver 102. The lens driver 102 includes a lens CPU and lens driving circuit, and outputs instructions to cause movement of the focus lens 101 to the focus position according to instructions from the control unit 109. The position of the focus lens 101a, that is, the focus position, is detected by the position detector 101a. 
The image-capture device driver 104 drives the image-capture device 103 that generates signals for photoelectric conversion of the image of the subject formed. In the image signal generator 105, electrical signals output from the image-capture device 103 are subjected to appropriate signal processing to generate image signals conforming to a prescribed standard. The image signals are transmitted to a circuit group (image signal processor 106) and are also input to the evaluation value calculator 107. The evaluation value calculator 107 filters high-frequency components of image signals in a specific region provided within a captured image frame, and computes an evaluation value corresponding to the contrast in the image. For subjects in general, the evaluation value increases as the focus lens approximates the point at which a subject image is in-focus state, and the evaluation value is relative maximum when the subject image is in-focus. This evaluation value is updated once for one field of image signals. In relation to auto-focus operations using evaluation values (evaluation value peak determination), one example is disclosed in Japanese Unexamined Patent No. 10-213736, filed by the present applicant.
The control unit 109 includes a CPU (Central Processing Unit) and the like, and receives evaluation values computed once for each field by the evaluation value calculator 107, and also receives distance measurement results from the distance measurement sensor 111 for an unspecified period, and uses these results (data) to perform an evaluation value peak search operation. The memory 110 is semiconductor memory or other nonvolatile memory, and stores the distance measurement results of the distance measurement sensor 111, focus positions of the lens 101, and other information.
The distance measurement sensor 111 includes an optical detection function and an output circuit, and measures the distance to a subject upon an instruction from the control unit 109 and outputs the result to the control unit 109. The range of existence of the subject is identified from the distance measurement results, that is, the approximate distance to the subject can be identified within a certain range.
By using the distance measurement information of the distance measurement sensor 111 and the evaluation value of the evaluation value calculator 107, while securing focus searching accuracy obtained through image processing, the focusing time when the focus lens is far from the focus position can be greatly decreased compared with image processing auto-focus operations. However, the distance measurement results of the distance measurement sensor 111 must always fall within a constant range of accuracy.
FIG. 2 shows the relation 1 between the distance to a subject before the changes occur in the distance measurement sensor 111 due to aging, and the distance measurement result of the distance measurement sensor 111. The accuracy of the distance measurement result has a constant width, and as shown in FIG. 2, the correspondence relation is described not by a line, but by a region 120 having a certain area. For example, when the distance measurement result is Ls, the distance to the subject cannot be identified uniquely, but as shown in FIG. 2, a subject existence range FJA within which the existence of the subject can be identified.
FIG. 3 shows an auto-focus operation 1 using distance measurement results. In FIG. 3, the evaluation value peak corresponds to the in-focus position FPj. First, when the focus position FPs at the start is more distant than a determination position LD1 from the subject existence range FJA based on the distance measurement result obtained by the distance measurement sensor 111, the motion velocity of the focus lens 101 is set to an ultra-high velocity Va which is the maximum velocity of lens driving. The motion direction is the direction toward the subject existence range FJA, and focusing operation is initiated. When the focus position FPs reaches the determination distance LD2 from the subject existence range FJA, the control unit 109 switches from the ultra-high velocity Va to a high velocity Vb. When the focus position FPs enters the subject existence range FJA, the focus position FPs is moved at a low velocity Vc such that an evaluation value, obtained by adding the frequency components of image signals in a specific region provided in a captured image frame, becomes relative maximum. In this way, by obtaining the subject existence range based on distance measurement results from the distance measurement sensor 111, the focus lens can be advanced at ultra-high speed Va from the near edge of the subject existence range FJA shown in FIG. 2 until a prescribed distance LD2 is reached, and as a result the focusing time can be shortened. Further, there is no need to perform wobbling and determine the motion direction due to changes in the evaluation value, so that a subject image free from blurring can be obtained rapidly with short focusing time.